I
think this is a more important question to ask than the (seemingly more common) threads relating to costs of bike/gear. Because if you're out there long enough, your equipment costs will end up being just a fraction of the total cost of touring.
Neil, I think you are absolutely right. We started to figure what our cross country trip cost (74 days, shipping bikes, airfare), and decided we did not want to know! However, from several lengthy tours, we generally figure between $50 and $90 a day for two of us. The price varies on the amount of meals cooked and the number of motels used. We have had a lot of $15 days, but on the other hand if you are in places like Victoria BC it is hard to stay under $150. I don't know about the rest of you, but we tend to eat about 4 meals a day (breakfasts 1 & 2) and an ice cream stop(s). I've also noticed that the cost seems to go up as I get older. I don't think I ever stayed in a motel on a tour before the age of 40.